Initial setting time is a significant index for characterizing the performance and development of cast concrete. For instance, during concrete roller compaction, the allowed pouring interval of upper and lower layers of concrete shall be less than the initial setting time; the surface finishing (secondary plastering) of fresh concrete must be completed between the initial setting time and the final setting time. Therefore, it is necessary to judge the setting time of the cast concrete in construction site.
The physical significance of setting is the solidification of plastic cement grout, initial setting means the beginning of solidification which indicates the loss of fluidity of mixed materials. The following determination methods are known currently:
Penetration resistance method: this is a method capable of directly testing setting time on a laboratory or engineering basis, and also a standard method for testing setting time. It is to essentially test the development of mechanical properties of cement grout from the macroscopic perspective and deliberately set certain criterion as the basis for judging setting time. For example, in the pin-penetration test of ASTM 403 and penetration resistance instrument test of GB 8076-87, the time when the penetration resistance reaches 3.5 MPa in test (the corresponding compressive strength is about 0) is defined as the initial setting time of concrete; penetration test in both ASTM C191 and GB 1346 is used to test the setting time of cement paste. This method has long been used, but shows many problems inevitably. When testing based on this method, firstly take a small amount of fresh concrete, remove coarse aggregate above 5 mm with a vibrating screen, place the remaining mortar in a mortar cylinder and then place the cylinder on the penetration resistance instrument regularly for test. This not only causes high cost in time and effort for test operation, in particular, leads to difficulty in removal of aggregate in some stiff concrete or accelerator mixed concrete. In addition, since testing point must be changed after once test, it is hard to realize automatic collection. More importantly, with this method, it is impossible to realize in-situ monitoring of structural concrete cast in construction site, and the setting time derived from small piece test cannot truly reflect the setting time of structural concrete as the setting time of concrete is subject to temperature, namely rising temperature may quicken the setting time, the temperature of the member's concrete is always higher than that of small test piece due to heat released from cement hydration in concrete of the structure after placement and its high structural volume. Hence, it is restrictive to accurately reflect the setting time of the structural concrete by penetration resistance method.
In addition to penetration resistance method for direct testing, hydration exotherm test method is also available: traditional hydration exotherm shows the hydration dynamic properties of cement grout, and hydration reaction leading to microstructure change; generally, initial setting corresponds to certain point after transition of latent period to acceleration period. Ultrasonic pulse velocity (UPV) test method: ultrasonic wave, especially shearing wave is difficult to be transferred in concrete of plastic stage due to loose structure; with the formation of the beginning of entanglement network structure, the system gradually transforms from suspension state to coagulation state, then the time capable of transferring shearing wave corresponds to the setting time of concrete. And electrical test method: including electric conductivity and electromotive force test methods, which are used for indirect testing by relying on the test of concrete's change of electrical properties (such as electric conductivity and electromotive force) with hydration and hardening. These methods are properly sensitive, but difficult in use and operation in construction site. Related testing indexes are prone to be subject to the interference of chemical ion in concrete mixture. More importantly, testing instrument has strict requirement for ambient conditions and the testing results may be affected by the change of temperature, humidity and noise, etc. Therefore, these methods are inapplicable to the in-situ test of setting time in actual engineering structural concrete.